The Synchrotron Peak Shift during High-Energy Flares of Blazars

A prediction for the energy shift of the synchrotron spectrum offlat-spectrum radio quasars (FSRQs) during high-energy flares is presented. Ifthe $\gamma$-ray emission of FSRQs is produced by Comptonization of externalradiation, then the peak of the synchrotron spectrum is predicted to move tolower energies in the flare state. This is opposite to the well-known broadbandspectral behavior of high-frequency peaked BL-Lac objects where the externalradiation field is believed to be weak and synchrotron-self Compton scatteringmight be the dominant $\gamma$-ray radiation mechanism. The synchrotron peakshift, if observed in FSRQs, can thus be used as a diagnostic to determine thedominant radiation mechanism in these objects. I suggest a few FSRQs aspromising candidates to test the prediction of the external-Comptonizationmodel.Comment: 9 pages, including 2 figures; uses epsf.sty, rotate.sty; accepted for ApJ Letters; minor revision